Pumps, compressors, agitators and valves are widely used in industries such as utilities, refineries, chemical and petrochemical facilities. While this equipment serves a variety of functions, each typically includes a rotary or reciprocating, motor-driven shaft. For example, the shaft of a rotary pump operatively connects a motor on the exterior of the pump casing to an impeller or blade on the interior of the casing. The motor rotates the shaft, which in turn, rotates the impeller. Fluid is pulled to the pump by the rotating impeller, passed across the impeller, and forced out of the pump under pressure. Thus, there are at least three openings in the pump (or valve) casing: (1) a first fluid opening for an inlet pipe; (2) a second fluid opening for an outlet pipe; and (3) an opening for the shaft.
The two fluid openings for the inlet and the outlet pipes are sealed conventionally. The shaft is sealed in a recessed area within the pump known as the "stuffing box". The stuffing box is located behind the impeller and in front of the opening in the pump casing through which the shaft projects. The term "stuffing box" is derived from the method employed to prevent fluid from leaking through the opening for the shaft; i.e. fluid is contained within the pump by stuffing or packing a material around the shaft to seal the opening. Thus, stuffing boxes in pumps, compressors, valves, etc. have the primary function of protecting the equipment against leakage at the point where the rotating or reciprocating shaft or the valve stem extends through the casing.
Rotary and reciprocating shaft equipped pumps, compressors, agitators and valves interact with a variety of fluids. Such fluids may be as harmless as cool water or as dangerous as a radioactive, superheated acid. Preventing leakage of any fluid from the opening for the shaft is, obviously, desirable. The cost of any such leakage can range from that associated with repairing the leak and replacing the fluid, to the unquantifiable cost of environmental damage or loss of life.
For example, consider a pump in a nuclear fueled steam generating plant. In nuclear reactors, pumps are used to circulate a coolant (oftentimes water) across nuclear fuel elements. The coolant and nuclear fuel are placed together in a pressure vessel. Piping from the pressure vessel delivers the coolant, heated by contact with the nuclear fuel, to a heat exchanger. The heat exchanger extracts the heat from the coolant. The coolant is then delivered once again to the pressure vessel to effect the cooling process. The piping thus forms a continuous loop between the pressure vessel and the heat exchanger so that the coolant is continuously recycled. As a result, radioactivity is safely contained within this closed system. Pumps are often provided between the pressure vessel and the heat exchanger to deliver the coolant. Any leakage from the pump destroys the closed system and permits radioactive coolant to escape. Failure of a seal in this example will not only result in the discharge of a toxic material into the environment, but could cause an explosion or fire.
Of course, the act of repairing such a leak carries many hidden costs. A nuclear reactor, like many other industrial processes requires significant time to regain normal operation. Any unnecessary shutdown of such a process greatly affects production capability. Thus, having to shut down a plant for any period of time in order to replace a failed packing reduces operating time and, correspondingly, profit.
Moreover, workers are often at risk in replacing failed packing. For example, a worker replacing failed (or worn) packing in a pump used to circulate coolant in a nuclear reactor incurs remarkable danger. The packing material in a pump and valve becomes saturated with the fluid with which it interacts. Thus, a worker replacing packing in a pump used to circulate coolant in a nuclear reactor will be exposed to radioactivity, since the coolant contains radioactive isotopes. A worker who removes failed packing from such a pump is, for a time, unavoidably exposed to radiation contained in the fluid. Accordingly, frequent replacement of the packing material in the stuffing box is not desirable. Moreover, it is preferred that all steps possible be taken to minimize the risk of such exposure and to minimize the length of time the worker is exposed.
Rotating and reciprocating shafts are difficult to seal because, in operation, the shaft is capable of both radial and axial displacement. Radial displacement typically results from manufacturing inaccuracies. Axial displacement results from different thermal expansions produced through normal operation of the shaft. The stuffing box environment is less than ideal. Conditions are constantly changing. Shaft speeds may vary. The packing may be required to withstand high temperatures and pressures one minute and low temperatures and pressures the next. The surfaces of the shaft in the stuffing box are often pitted and rough, causing excessive and uneven wear of the packing material. Those skilled in the art will appreciate that very slight defects in the arrangement or condition of a stuffing box can prevent proper pump operation.
In addition, those skilled in the art will appreciate that friction between the shaft and the packing produces heat. The more the shaft is pitted and roughed, the greater the coefficient of friction becomes between the shaft and the packing; accordingly, more heat is generated. Excessive heat can cause the packing to harden and lose resiliency, thereby creating spaces and gaps where leakage can occur between the packing and the shaft. Excessive friction can itself cause a valve to fail. In some instances, valve failure has resulted in a plant wide shut down. Consequently, the Nuclear Regulatory Commission ("NRC"), an administrative agency of the United States government, has recently undertaken an investigation of the effect of valve stem friction. Thus, the problem of stem friction is recognized as substantial. The packing must be made of a material that minimizes the inherent friction between the shaft and the packing.
To the greatest possible extent, the packing material must make accommodations for each of these factors. The packing must be somewhat plastic so that it can be adjusted for proper operation. The packing must resist excessive extrusion, but expand enough to seal rough or uneven surfaces. The packing must be resilient in order to adapt to changing conditions without failing or damaging the shaft.
Various types of packing for a stuffing box are known in the prior art. Each of these packings attempt to be responsive to the foregoing considerations. However, in trying to provide flexibility, some packings sacrifice resiliency. Others, in trying to resist extrusion, sacrifice flexibility sufficient to conform to uneven and/or rough surfaces within the stuffing box. Still other packings are flexible, resilient and minimize friction, but do not provide a long-lasting seal so as to avoid frequent replacement.
Soft packing is a common shaft seal, and is generally made from asbestos, fabric, hemp or rubber fibers woven into strands and formed into a braided length. (It is to be understood that asbestos is being phased out as a packing material due to the well known health hazards posed by this material.) Soft packing is inexpensive and offers several desirable features. The softness of the packing allows it to absorb energy without damaging the rotating shaft. Soft packing is also very flexible and readily conforms to the area to be sealed.
Soft packing, however, has several disadvantages. One problem is short life. Soft packing is easily worn by friction and easily damaged, therefore requiring frequent replacement. Soft packing may be impregnated with graphite or lubricating oils to reduce friction between the shaft and the packing, but such lubricants quickly dissipate and are not very effective in overcoming the short life problem. Thus, soft packings are best suited for low shaft speed applications involving non-caustic and non-abrasive fluids. Further, soft packings are poorly suited for high temperatures and pressures. At high temperatures, fiber breakdown occurs. At high pressures, soft packings extrude excessively. Yet another problem with soft packing is a lack of resiliency. After being compressed and extruded, soft packings are unable to re-expand to effectuate a seal. Resiliency, conventionally defined as the ability of packing to re-expand, is important to enable the packing material to adjust to changing conditions. Lack of such resiliency, as in the case of a soft packing, results in frequent adjustment or replacement for the packing.
Another type of packing is metallic packing. Metallic packing is made similarly to soft packing, but incorporates flexible metallic strands or foils. In addition, metallic packing is often provided with an asbestos or plastic core. Metallic packings are not as flexible as soft packings, but are much more resilient. The resiliency of the metallic packing resists excessive extrusion and permits use in higher pressures. The metallic strands and foils are more resistant to breakdown due to high temperature than soft packings. Metallic packing suffers from the inability to minimize friction. While it is known to add lubricants, they eventually wear away. As with soft packings, metallic packings can be destroyed by friction, thereby resulting in leaks and damage to the shaft.
A known stuffing box packing is made from expanded graphite tape. More specifically, a known packing provides a length of flexible expanded graphite tape that is wound about a mandrel to form a solid annulus of appropriate size. Thus, the expanded graphite tape is itself formed as a seal and packed into the stuffing box. Packing made from expanded graphite is flexible and conforms to uneven surfaces. The graphite material makes the packing self-lubricating, thereby minimizing friction between the shaft and the packing. With such self-lubricating packings, the lubricant does not dissipate with time. Expanded graphite packing also absorbs energy without excessive damage to either the packing or the shaft.
The principal problems with expanded graphite packings are threefold. First, expanded graphite packings lack resiliancy, resulting in an inability to adjust to changing conditions within the stuffing box. Second, expanded graphite packings extrude excessively under high temperatures, resulting in leaks and premature replacement. Thurd, expanded graphite packings are difficult to remove from a stuffing box because the packing is easily pulled apart by the hook or like tool that is inserted into the stuffing box to pull the packing material. Solid graphite packings are not able to withstand high pressures since they lack the internal strength to resist extrusion and are unable to re-expand after compression. In addition, expanded graphite packings require frequent adjustment under normal conditions due to the low resiliency of the graphite. The graphite packings are easily compressed, thereby contributing to the low resiliency problem. As a result, normal rotation or reciprocation of the shaft can compress the graphite and create leaks.
U.S. Pat. No. 4,667,969 teaches spirally winding expanded graphite foil tape in an overlapping manner about a resilient, flexible core of longitudinally braided yarns. Thus, this patent teaches a combination of soft packing together with an expanded graphite packing. The expanded graphite foil tape forms an energy absorbent, self-lubricating skin about the core. The core adds internal strength to resist extrusion and increases resilience to encourage re-expansion after compression.
Even so, combining a soft type of packing with an expanded graphite type of packing fails to provide any of the advantages presented by a metallic packing. As noted above, metallic packings are more resistant to high temperature breakdowns. Thus, the preferred packing would provide such high temperature stability.
Another principal advantage of metallic packing is the ease with which it can be removed from a stuffing box. Conventional removal of a packing material is accomplished with a hook or like device. Ideally, the hook is inserted into the stuffing box, secured within the packing to be removed, and pulled out of the stuffing box, pulling the packing material. therewith. Soft packing is difficult to remove because the material pulls apart. A soft packing material wrapped with an adhesive-backed flexible expanded graphite tape is also difficult to remove because the adhesive may catch the shaft or the valve stem. Because the adhesive is also bonded to the soft core, the entire packing may become lodged within the stuffing box. Metallic packing is easier to remove because the hook can latch onto one of the metallic strands (which is in turn interwoven with other metallic strands) such that the entire packing can be readily pulled from the stuffing box. This feature of metallic packings is particularly important in terms of reducing the amount of time required to replace worn or failed packing. Taking the nuclear power plant example given hereinabove, it will be appreciated that every second a worker is needlessly exposed to radiation is to be eliminated. When a soft packing disintegrates within a stuffing box, the worker spends an inordinate amount of time removing the packing section-by-section--resulting in an inordinate amount of exposure to radiation. Because metallic packing is far less likely to disintegrate and much easier to remove, the worker is subjected to much less radiation.
A great deal of attention in stuffing box seals has focused on the core element. For example, it is known to provide a soft flexible core made of a yarn or like fiber. Such a core is beneficial in that, like soft packings, a soft core is inexpensive, readily conforms to the stuffing box, and absorbs energy without damaging the shaft. It is further known to provide a core of metal mesh core. Metallic cores, like metal packings, are very effective under high pressure and high temperature conditions. Metallic cores are also easy to remove from a stuffing box. However, metallic cores are unable to minimize friction. This problem is compounded by the fact that lubricants cannot be added under conventional technology to a metallic core. Thus, no lubricant could be used to reduce the coefficient of friction introduced by a metallic core element. U.S. Pat. No. 4,219,204 further shows that it is known to provide a combination of discrete sections made of different materials. More specifically, U.S. Pat. No. 4,219,204 shows a sealing device that incorporates anti-extrusion sections formed of knitted materials which, in turn, incorporate some metallic wires or filaments.
Those skilled in the art will appreciate that a stuffing box seal defines an inside and an outside diameter. The inside diameter is defined according to the shaft or valve stem. The outside diameter is defined according to the wall of the stuffing box. Conventional seals come in long lengths of varied cross-sections; i.e. 3/8", 1/2", 5/88", etc. It is industry practice to buy a length of a packing material and cut the material to a predetermined, shorter length, from which to form a single, ring-like seal. Formation of the seal is conventionally accomplished by die-molding. The cut length of packing material is placed in a die, where it is compressed to a desired density and to the appropriate inside and outside diameter specifications. The molded ring is then provided to the end-user where it is installed into the stuffing box. The alternative to this die-molding operation is to order a length of packing material and utilize the stuffing box itself to achieve the appropriate density. According to this alternative, the length of packing material is cut and a ring formed. The ring is inserted into the stuffing box and the gland follower secured to compress the sealing ring to an appropriate density. The gland follower is then removed to permit introduction of the next sealing ring and the process repeated for that seal ring; and each of the additional sealing rings to be installed. Packing material of varying cross sections and diameters are stocked to provide replacement seals when needed. The method carries the obvious disadvantages of being inaccurate in terms of compression and slow to number of repetitions that must be undertaken with the gland follower.
The prior art therefore lacks a core element that combines the benefits of graphite with the benefits of a metallic core. There is a need for a packing material that combines the benefits of a metallic packing material with those of soft and expanded graphite types of packing materials. Stated otherwise, there exists a need for a packing material formed with a core that is sufficiently resilient and self-lubricating but resists extrusion; that provides a core element that performs well under both high temperatures and high pressures; but returns the benefits of soft cores and metallic cores. Further, there is a need for a seal that provides the benefits of a previously formed seal but avoids the need to stockpile a variety of seals having different inside and outside diameters. Yet further, there is a need for a seal that omits the die-molding step of conventional installation so as to reduce the time required to replace an old or worn out seal. Finally, there exists a need in the art for an economical, simple method of manufacturing such a packing.